1. Field of the Invention
The present invention relates to electronic circuits and, in particular, to a power FET gate discharge circuit which remains active after all other circuitry is turned off to fully discharge the power FET gate and then turns itself off so that only a single low current path remains between supply and ground.
2. Discussion of the Prior Art
The gate impedance of a large power MOSFET transistor is highly capacitive. Hence, the terms "charge" and "discharge" are used to describe turning the power transistor on and off, respectively.
In all high-side driver applications (and in low-side drivers operating from low supply voltages), the gate of the power FET must be raised above the supply potential in order to turn the power FET on. This requires the use of charging circuitry which "pumps" the power FET gate above the supply to the desired potential. Typically, this charging circuitry is designed to maintain the desired potential on the power FET gate after the FET has been turned on while at the same time minimizing extraneous current drain.
Similarly, discharge circuitry is required to pull the gate of the power FET down to turn the FET off. Optimally, the discharge circuitry should conserve current when the power FET is off. For example, in automotive applications, it is required that some circuitry (e.g. clock, memory radio, etc.) remain powered even when the ignition is turned off; it is highly desirable in this application to provide power to these selected circuits while at the same time minimizing the current drain on the car battery.
Typically, MOS circuitry is utilized for implementing charging and discharging circuitry for power FETs because of the true on-off and high speed switching capability of MOS transistors.
However, a major deficiency of MOS switches is that, in some harsh operating environments, the low breakdown voltages of the MOS transistors are insufficient for the transistors to withstand high voltage transients. For example, in automotive systems, the possibility of inadvertent reverse battery conditions or loose battery cables has caused some automotive manufacturers to specify that integrated circuits (IC) utilized in these applications be capable of withstanding up to 60V, well above the breakdown voltage of commonly available MOS transistors. Thus, additional protective circuitry is required to shield the MOS switching transistors from voltage transients.
Discharging circuitry can be implemented utilizing bipolar transistors configured to withstand high voltage transients. However, substantial bias currents are typically required in order to maintain desired on/off switching speeds.